This paper examines the practice of collision avoidance in Merchant ships to highlight the role of tacit knowledge and collective learning in the formation of professional practice. Analysis of 1,431 recorded VHF radio conversations drawn from real-time ship-to-ship interactions provides a unique window onto professional competence in navigation, and the application of professional judgement underpinned by tacit knowledge learned through practice. Moreover, we argue that such tacit knowledge is gained within a 'community of practice'. More specifically, our data reveal how: (i) the practice of collision avoidance is more collaborative than reference to the extant regulations (the COLREGs) alone would suggest, and (ii) the practice of discussing collision avoidance over the radio can be seen as a form of collective production and learning of tacit knowledge through practice. Consequently, our analysis provides insight into the social nature of collision avoidance practice. In so doing, the paper makes a significant contribution by (i) adding a new example to the corpus of research in this area, (ii) introducing the community perspective of learning to the maritime sector, (iii) extending the concept of community-based learning by presenting data that illustrates both individual and collective learning.